(180 kDa) membrane receptor for secretory phospholipases A2

of the physiological effects of group I SPLA2, including smooth muscle contraction and cell ... group II SPLA2S as well as various SPLA2S purified fro...
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Biochemistry 1995, 34, 13146-13151

13146

Multifunctional Activity of the Extracellular Domain of the M-Type ( 180 kDa) Membrane Receptor for Secretory Phospholipases A2t Philippe Ancian, GCrard Lambeau, and Michel Lazdunski" Institut de Pharmacologie MolCculaire et Cellulaire, 660 route des Lucioles, Sophia Antipolis, 06560 Valbonne, France Received June 22, 1995; Reuised Manuscript Receiued August 14, 1995@

M-type (180 kDa) receptors for secretory phospholipases A2 (sPLA2s) are thought to mediate some of the physiological effects of group I sPLA2, including smooth muscle contraction and cell proliferation. The M-type sPLA2 receptor is a large glycoprotein composed of several distinct extracellular domains which belongs to the C-type lectin superfamily. This receptor binds with high affinity both pancreatic group I and inflammatory group I1 sPLAzs as well as various sPLA2s purified from snake venoms. This paper shows that the rabbit M-type sPLA2 receptor is a multifunctional protein which is able to promote cell adhesion on type I and IV collagens most probably via its N-terminal fibronectinlike type I1 domain. It also shows that binding of sPLA2s to a recombinant soluble form of this receptor is associated with a noncompetitive inhibition of phospholipase A2 activity. ABSTRACT:

Secretory phospholipases Az (sPLA2s)I are structurally related enzymes which catalyze the hydrolysis of the acylester bond at the sn-2 position of the glycerophospholipids (Dennis, 1983). These enzymes are found in mammals as well as in insect and snake venoms (Waite, 1987; Mayer & Marshall, 1993; Dennis, 1994). Snake venom PLA2s (svPLAzs) produce neurotoxic, myotoxic, anticoagulant, and proinflammatory effects (Kini & Evans, 1989; Hawgood & Bon, 1991). This diversity of effects seems to be linked to the existence of very high affinity receptors for svPLA2s that have been previously identified in brain (Lambeau et al., 1989) as well as in other tissues (Lambeau et al., 1990, 1991a,b). Two main types of mammalian sPLA2s have been extensively studied. The inflammatory group I1 sPLA2 is considered as a potent mediator of the inflammatory process. It is found in plasma and synovial fluids of patients with various inflammatory diseases (Vadas & Pruzanski, 1986; Mukherjee et al., 1992; Kudo et al., 1993; Pruzanski et al., 1993) and has been proposed to play a key role in the pathogenesis of these diseases. This enzyme is up-regulated by proinflammatory cytokines like interleukin- 1, interleukin6, and tumor necrosis factor (Vadas & Pruzanski, 1986; Mukherjee et al., 1992; Kudo et al., 1993; Pruzanski et al., 1993) and is implicated in the production of potent lipid mediators of inflammation (Barbour & Dennis, 1993; Murakami et al., 1993; Suga et al., 1993; Fourcade et al., 1995). The pancreatic-type sPLA2 has been first implicated in the This work was supported by the Centre National de la Recherche Scientifique (CNRS), the Association pour la Recherche sur le Cancer (ARC), and the Ministtre de la DCfense Nationale (Grant DRET 931 122). * To whom correspondence should be addressed at the Institut de Pharmacologie Moltculaire et Cellulaire, 660 route des Lucioles, Sophia Antipolis, 06560 Valbonne, France. Telephone: 33 93 95 77 02 or 00. Fax: 33 93 95 77 04. E-mail: [email protected]. Abstract published in Advunce ACS Abstracts, September 15, 1995. I Abbreviations: PLA2, phospholipase A*; sPLA2, secretory phospholipase A?; svPLA2, snake venom phospholipase Az; OS,, toxin 1 from Oxyuranus scutellatus scutellatus; OS*, toxin 2 from Oxyuranus scurellatus scutellatus; CRD, carbohydrate recognition domain. +

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0006-2960/95/0434- 13146$09.00/0

digestion of dietary phospholipids (De Haas et al., 1968) and more recently in cell proliferation (Arita et al., 1991) and smooth muscle contraction (Nakajima et al., 1992; Sommers et al., 1992). These latter effects are thought to be linked to the existence of high affinity receptors with molecular masses of 180-200 kDa (M-type sPLA2 receptors) which have been characterized in rabbit (Lambeau et al., 1994), rat (Lambeau et al., 1991a; Hanasaki & Arita, 1992), bovine (Ishizaki et al., 1994), and human tissues (Ancian et al., 1995). The rabbit M-type sPLA2 receptor binds with a high affinity both the porcine pancreatic and the human inflammatory group I1 sPLAzs, as well as OS1 and OS2, two svPLA2s purified from the venom of the Taipan snake Oxyuranus scutellatus scutellatus (Lambeau et al., 1994). Structure-function relationship studies with the porcine pancreatic sPLA2 have identified the sPLA2s residues which are crucially involved in the binding to the rabbit M-type sPLA2 receptor (Lambeau et al., 1995). Molecular cloning of M-type sPLA2 receptors (Lambeau et al., 1994; Ancian et al., 1995) has revealed that they share a common structural organization with the macrophage mannose receptor, a transmembrane C-type lectin involved in the endocytosis of glycoproteins and the phagocytosis of pathogenic microorganisms bearing mannose residues on their surface (Ezekowitz et al., 1990; Taylor et al., 1990). The extracellular domain of the M-type sPLA2 receptor is large (x1400 residues) and composed of several distinct domains, including a N-terminal cystein-rich domain, a fibronectin-like type I1 domain, and eight carbohydrate recognition domains (CRDs) in tandem, followed by a single transmembrane segment and a short cytoplasmic tail. A transcript encoding for a soluble M-type sPLA2 receptor lacking the transmembrane segment and the cytoplasmic tail has been identified in human kidney (Ancian et al., 1995). This paper shows that the rabbit M-type sPLA2 receptor is a multifunctional protein with an extracellular domain which is able to promote cellular adhesion on type I and type IV collagens and to inhibit the sPLA2 catalytic activity upon binding.

0 1995 American Chemical Society

Multifunctional Role of the M-Type sPLA2 Receptor

MATERIALS AND METHODS Materials. 0.scutellatus scutellatus toxins (OS1 and OS2) were purified as previously described (Lambeau et al., 1989). The porcine pancreatic sPLA2 (group I) was purchased from Boehringer Mannheim. The human inflammatory sPLA2 (group 11) was a generous gift from Dr. Ruth Kramer (Eli Lilly Co., Indianapolis, IN). It is a recombinant protein expressed in Syrian Hamster AV 12 cells. Human type I and type IV collagens (catalog no. C7774 and C7521, respectively) were purchased from Sigma. Construction of Rabbit M-type sPLA2 Receptor Mutants. Mutants of the rabbit M-type sPLA2 receptor were obtained by site-directed mutagenesis according to the methyl-dCTP method (Vandeyar et al., 1988). For this purpose, the cDNA encoding for the rabbit M-type sPLA2 receptor was shortened by removing 713 bp in the 3' noncoding region, between two SexAl sites located at positions 4556 and 5269 (Lambeau et al., 1994), and subcloned into the expression vector pRc/CMV (Invitrogen). Single-stranded DNA was produced using R408 helper phage in Escherichia coli JM 101 cells. Deletions were obtained by the gapped duplex method (Kramer et al., 1984) with 40-mer oligonucleotides mutagenic primers. ANF is a modified form of the rabbit M-type sPLA, receptor which lacks the N-terminal cystein-rich domain as well as the fibronectin-like type I1 domain [residues 1-2 18 (Lambeau et al., 1994)]. ANF was obtained with the oligonucleotide 5'-GGGCAGAATCCCCACTTCTCCCACTTGAGGACCCGCTCGG-3'. ATSM is a form of the receptor which lacks the transmembrane segment [residues 13941416 (Lambeau et al., 1994)] and then corresponds to a soluble rabbit M-type sPLA2 receptor. It was constructed using the oligonucleotide 5'-GCAAGTCTCCTGAAGAATCCGTGAATATCTGCCTTCATTT-3'. Stable Expression in 293 Cells. cDNAs encoding for wildtype and mutated forms of the rabbit M-type sPLA2 receptor were stably transfected into 293 cells (American Type Cell Collection) by the Cap04 procedure (Graham & Van der Erb, 1973). Fifty percent confluent cells were transfected with 20 p g of DNN75 cm2 Petri dish on day 1. On day 2, the cells were trypsinized and replated. Transfected cells were selected in a medium supplemented with 1 mg/mL G418. Individual resistant colonies were selected and assayed for [1251]OS I binding as previously described (Lambeau et al., 1990). All the binding experiments were performed in a buffer containing 140 mh4 NaC1,50 mM Tris, pH 7.4, 1 mM CaC12, and 0.1% BSA. For binding experiments on the soluble recombinant rabbit receptor, the serum-free medium from transfected cells was centrifuged at lOOOOg for 10 min. Aliquots of the supernatant were assayed for [1251]OS~ binding as described above and filtered through Whatman G F F glass-fiber filters. The concentration of the soluble M-type sPLA2 receptor has been determined with [1251]OS1as a ligand. For the inhibition of the sPLA2 activities, serum-free medium containing the soluble M-type sPLA2 receptor was concentrated by centrifugation using Centricon 30 (Amicon). Mock-transfected cells were obtained by transfection with the parent vector pRC/CMV. Immunoblotting. Transfected 293 cells were rinsed three times with PBS, scraped in 20 mM Tris-HC1, pH 7.5, and 2 mM EDTA and sonicated. Cell homogenates and serumfree extracellular medium were analyzed by SDS -PAGE in 7.5% acrylamide gels. Protein samples were transferred

Biochemistry, Vol. 34, No. 40, 1995 13147 to nitrocellulose (Hybond C-extra, Amersham). Nitrocellulose sheets were blocked with 3% nonfat drymilk in PBS and incubated with guinea pig polyclonal antibodies raised against the purified whole rabbit M-type sPLA2 receptor (working dilution 1:5000),followed by peroxidaseconjugated goat anti-guinea pig IgG (Cappel Research Products; working dilution 1:10000). After extensive washings in PBS containing 0.1% Tween 20, blots were revealed with the BM chemiluminescence western blotting reagent (Boehringer Mannheim) and exposed to X-OMAT AR films (Eastman Kodak Co.). Cell Adhesion Assays. 96-well plates were coated for 1 h at 37 "C with laminin, type I collagens from rat tail or from human placenta, or type IV collagen from human placenta at 20 pglmL in PBS. Wells were washed twice with PBS and blocked for 1 h at 37 "C with 0.1% BSA in PBS. Wells were washed again twice with PBS. Stably transfected 293 cells expressing the wild-type or the mutant ANF rabbit M-type sPLA2 receptor were harvested by mild trypsinization in the presence of EDTA, centrifuged, and resuspended in serum-free medium containing 0.1% BSA. Immunoblotting experiments have shown that the M-type sPLA2 receptor was not affected by the mild trypsinization procedure (not shown). To measure inhibition of the attachment, cells were resuspended in serum-free medium containing 100 nM recombinant soluble M-type sPLA2 receptor or the equivalent amount of serum-free medium of mock-transfected cells. A total of 5 x lo4 cells were added to each well and allowed to attach during 15 min at 37 "C. Nonadherent cells were removed by gentle washing once with 200 p L of PBS. Adherent cells were stained overnight with PBS containing 1% toluidine blue and 3% paraformaldehyde. After extensive washings with PBS, stained cells were dissolved in 50 p L of 2 M, HCI and the optical density of each well was measured at 570 nm. A linear relationship was observed between the optical density and the number of fixed stained cells. This relationship was identical for each cellular clone. sPLA2 Activity. sPLA2 activity was measured by the hydrolysis of autoclaved [3H]oleate-labeled E. coli membranes as substrate (Franson et al., 1974). Briefly, 1 mL of a overnight culture of E. coli XL1 strain (Stratagene Inc.) was incubated for 4 h at 37 "C in 75 mL of LB medium containing 500 pCi of [3H]oleic acid (NEN). Cells were pelleted for 10 min at 3000g, resuspended in 50 mL of LB medium, and allowed to chase for 30 min at 37 "C. After centrifugation, the cell pellet was washed in 1 mL of 0.1 M Tris, pH 8, and 1% free fatty acid BSA, centrifuged, resuspended in 2 mL of 140 mM NaCl and 20 mM Tris, pH 7.4, and autoclaved. The reaction mixture (100 pL) contained 140 mM NaC1, 50 mM Tris, pH 7.4, 1 mM CaC12, 0.1% BSA, various amounts of [3H]oleate-labeled E. coli membranes, and sPLA2s as described in the legends to figures. Reaction mixtures were incubated for 2 min at 25 "C and then stopped by adding 300 p L of a solution containing 0.1 M EDTA, pH 8, and 1% free fatty acid BSA. After centrifugation at lOOOOg for 3 min, aliquots of supernatants containing hydrolyzed phospholipids were counted. For the inhibition of catalytic activity, sPLA2s were preincubated for 90 min at 25 "C with various concentrations of the recombinant soluble rabbit M-type sPLA2 receptor, after which substrate was added in a small volume (10 pL) and sPLA2 activity measured as above.

Ancian et al.

13148 Biochemistn, Vol. 34, No. 40, I995 0.5

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FIGURE 1 : Immunoblot of native and truncated rabbit M-type sPLA2 receptors. Protein samples were separated on a 7.5% acrylamide gel, electroblotted, and subjected to an immunolabeling using an anti-rabbit M-type sPLA2 receptor guinea pig antiserum. PLA2-R clone 1 and clone 2: membranes of cell stably transfected with the wild-type rabbit M-type sPLA2 receptor (see text). ANF: membranes of cell stably transfected with a truncated rabbit M-type sPLA2 receptor lacking the N-terminal cystein-rich domain as well as the fibronectin-liketype I1 domain. ATSM: serum-free medium of cells stably transfected with a truncated soluble receptor lacking the transmembrane segment. Mock: Membranes of mock-transfected cells. SFM PLA2-R: serum-free medium of PLA2-R clone 2. To ensure that all kinetics measurements were conducted under initial rate conditions, time course studies were carried out using [3H]oleate-labeled E. coli as substrate. Using human group I1 sPLA2 at a final concentration of 0.3 nM, results indicate that phospholipids hydrolysis was linear through 2 min at 25 "C and that the reaction velocity became constant after a lag period of 20 s. Hydrolysis after 2 min was less than 10% of the total phospholipids. Similar results were obtained using OS2 at 5 nM and the porcine pancreatic and the bee venom sPLA2s at 0.5 nM (not shown).

RESULTS AND DISCUSSION One of the characteristic features of the M-type sPLA2 receptor is the presence in the N-terminal extracellular region of a fibronectin-like type I1 domain. This domain is also found in other proteins such as fibronectin (Skorstengaard et al., 1986), the bovine seminal protein PDC-109 (Esch et al., 1983), and the type IV collagenase (Collier et al., 1988). In all these proteins, this domain has been shown to contain collagen-binding determinants (Banya et al., 1990; Banya & Patthy, 1991; Skorstengaard et al., 1994). In order to check whether the fibronectin-like type I1 domain of the M-type sPLA2 receptor can fulfill the same function, we produced stable transfectantsexpressing the native full-length M-type sPLA2 receptor or a truncated receptor lacking the N-terminal cystein-rich and the fibronectin-like type I1 domains (called ANF). Western-blot analysis using guinea pig anti-rabbit M-type sPLA2 receptor antiserum revealed that one major band of 180 kDa and one minor band of 160 kDa were labeled in the membranes of cells stably transfected with the full-length cDNA (PLA2-R clones 1 and 2, Figure 1). Two bands of 170 and 150 kDa were labeled in membranes of cells stably transfected with the truncated cDNA (ANF, Figure 1). All these bands were specifically labeled since no signal was obtained with nonimmune serum antibodies (not shown). The nature of the 160 and 150 kDa bands is presently unknown. One possibility is that they correspond to incompletely processed M-type sPLA2 receptors, with different degrees of glycosylation. Further experi-

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FIGURE 2: Adhesion of cell clones expressing the native or a truncated M-type sPLA2 receptor. A total of 5 x 104 cells were seeded in 96-well plates precoated with various collagens at 20 pg/mL in PBS. Mock shows the attachment of mock-transfected 293 cells, and ANF represents the adhesion of a 293 cell clone expressing a truncated rabbit M-type sPLA2 receptor lacking the N-terminal cystein-rich domain as well as the fibronectin-like type I1 domain with a Bmax value of 1. I pmol/mg of protein for ['2sI]OSI. PLA2-R clone 1 and clone 2 show the adhesion of 293 cell clones expressing the full-length receptor with Rmax values of 0.16 and 7 pmol/mg of protein for [12sI]OS~, respectively. Signs - and + on the bottom of the figure indicate the absence or the presence of 100 nM of the soluble M-type sPLA2 receptor in the incubation medium. Each measure is the mean f SEM for n = 3. ments are needed to address this question. PLA2-R clones 1 and 2 expressed the full-length rabbit M-type sPLA2 receptor with maximum binding capacity (Bmax) values of 0.16 and 7 pmol/mg of protein, respectively, for [12sI]OSI (a specific ligand of the M-type sPLA2 receptor) (not shown). The truncated M-type sPLA2 receptor retained the same binding properties as the full-length receptor (not shown). The ANF clone expressed the mutated receptor with a Bmax value for [12sI]OSIof 1 . 1 pmol/mg of protein (not shown). Figure 2 shows that cells expressing the full-length M-type sPLA2 receptor attach to collagen while mock-transfected cells and ANF do not. Adhesion to collagens seems to be related to the expression level of the M-type sPLA2 receptor as the PLA2-R clone 2 which expresses higher levels of the M-type sPLA2 receptor (a higher Bmax value) than PLA2-R clone 1 attaches better on the different types of collagen. Cellular attachment is specific to type I and type IV collagens. A negative control was made on plates precoated with laminin. On this substrate, no cells were seen to adhere after 15 min. The inhibition of adhesion by a recombinant soluble M-type sPLA2 receptor has been assayed to ensure that the attachment of cells to collagen was mediated by the M-type sPLA2 receptor. In order to produce this soluble form of the M-type sPLA2 receptor, a mutant cDNA which lacks the sequence encoding for the transmembrane segment has been constructed by site-directed mutagenesis. 293 cells have been stably transfected with this modified cDNA, and one clone (called ATSM) has been isolated after G418 selection. Western-blot analysis using guinea pig anti-rabbit M-type sPLA2 receptor antiserum revealed the presence of a 180 kDa band in the supernatant of ATSM (Figure 1). No band was detected in the medium of mock-transfected cells

Biochemistry, Val. 34, No. 40, 1995

Multifunctional Role of the M-Type sPLA2 Receptor or in the medium of cells stably transfected with the cDNA encoding for the transmembrane form of the rabbit M-type sPLA2 receptor [mock and SFM PLA2-R, respectively (Figure l)]. When PLA2-R clones 1 and 2 were incubated with 100 nM ATSM on plates precoated with collagen, cellular attachment was drastically reduced (Figure 2 ) , to a level comparable to that of the mock-transfected cells. No inhibition was observed when PLA2-R clones 1 and 2 were incubated with serum-free medium of mock-transfected cells (not shown). All these data taken together clearly indicate that the N-terminal region of the M-type sPLA2 receptor has adhesive properties to type I and IV collagens. The functional significance of these properties in relation to the physiological role of the M-type sPLA2 receptor still has to be determined. Of the two domains which have been deleted in the ANF mutant, the fibronectin-like type I1 domain is most probably directly involved in cellular adhesion to collagen since fibronectin-like domains in other proteins are known to bind to collagens (Banya et al., 1990; Banya & Patthy, 1991; Skorstengaard et al., 1994). One of the key questions related to the M-type sPLA2 receptor is to know whether it alters sPLA2 activity. This problem has been solved using the cell clone expressing the truncated soluble ATSM receptor. This clone expressed the soluble M-type sPLA2 receptor at a very high level [ 1.2 pmoV lo6 cells/24 h, measured with [1251]OS~ as a ligand (not shown)]. The soluble receptor accumulated in the medium until 10 days (not shown). Saturation curves with [1251]OS1 as a ligand revealed that the recombinant soluble M-type sPLA2 receptor recognizes this svPLA2 with a Kd value of 40 pM (Figure 3A). Competition experiments showed that OS2 inhibits [1251]OS1binding to the soluble M-type sPLA2 receptor with a KO5 value of 30 pM (Figure 3B) and that the porcine pancreatic and the human group I1 sPLA2s inhibit [1251]OS1binding with KO.5 values of 30 and 3 nM, respectively (Figure 3B). Conversely, the bee venom sPLA2 is without effect on [1251]OS1binding (Figure 3B). These KO.5 values are very similar to those measured for the wild-type transmembrane rabbit M-type sPLA2 receptor (Lambeau et al., 1994). Figure 4 shows the inhibitory effects of the recombinant soluble M-type sPLA2 receptor on the enzymatic activities of several sPLA2s. Figure 4A shows that OS2 associates stoechiometrically with the soluble M-type sPLA2 receptor. Activities of the human group I1 and porcine pancreatic sPLA2s are inhibited with IC50 values of 2.1 and 14.4 nM, respectively (Figure 4B). The bee venom sPLA2, which does not bind to the rabbit M-type sPLA2 receptor, is not inhibited by this receptor protein (Figure 4B). Figure 4C shows double-reciprocal plots of kinetic data for the hydrolysis of [3H]oleate-labeled E. coli by the human group I1 sPLA2 at different soluble M-type sPLA2 receptor concentrations. Lineweaver-Burk analysis reveals that the soluble M-type sPLA2 receptor behaves as a noncompetitive inhibitor (V,,, is changed, K, is unchanged) of the human group I1 sPLA2 with a calculated K, value of 1.4 nM. The M-type receptor for sPLA2s contains eight CRDs in tandem, and site-directed mutagenesis experiments have recently shown that sPLA2s bind to the M-type receptor via its CRDs (article in preparation). The fact that the M-type receptor is inhibitory for sPLAzs is to be put in parallel with the recent identification in snake plasma (Inoue et al., 1991; Ohkura et al., 1993) as well as in mammals (Fisher et al.,

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FIGURE3: Expression of the recombinant soluble rabbit M-type sPLA2 receptor in stably transfected 293 cells. (A) Equilibrium binding of [1251]OS1to serum-free medium of cells stably transfected with a mutated rabbit M-type sPLAz receptor lacking the transmembrane segment (ATSM). (Main panel) Saturation curve with [1251]OS1obtained by difference between total and nonspecific binding measured in the presence of 30 nM unlabeled OS I . (Inset) Scatchard plot analysis of the specific binding. (B) Competition experiments with [1251]OS1(10 pM) and different unlabeled sPLA2s for the binding to the recombinant soluble rabbit M-type sPLA2 receptor. Results are expressed as percentage of the maximal specific binding measured in the absence of competitor. 100% corresponded to 2 pM [1251]OSispecifically bound. Nonspecific binding was measured in the presence of 30 nM unlabeled OS1 and represented 20% of the total binding. No binding activity was detected either with the medium of mock-transfected cells or with the medium of cells transfected with the membrane-anchored receptor cDNA.

1994) of several proteins which also behave as noncompetitive inhibitors of sPLA2 activity and which comprise a CRDlike domain. The inhibitory action of the M-type sPLA2 receptor is not really surprising since residues that have been previously shown to be essential for binding of the pancreatic sPLA2 to M-type sPLA2 receptors such as Gly-30 and Asp49 (Lambeau et al., 1995) are also essential for the catalytic activity of the enzyme (Verheij et al., 1980). One of the functions of the sPLA2 receptor is then clearly to neutralize the enzymatic activity of its ligand. Whether this property is an essential component of the transduction system associated with the sPLA~/M-typereceptor interaction is not known. On the other hand, soluble forms of the human sPLA2 receptor have been identified (Ancian et al., 1995). Since there has been over the last years a long search of inhibitors of type I1 sPLA2 with the idea that they could be used in inflammation and against rheumatoid arthritis (Wilkerson, 1990; Gelb et al., 1994), the production of recombinant forms of these receptors might then lead to useful therapeutic applications. Similarly, the use of the soluble forms of receptors for cytokines has been proposed as a possible way to prevent their action in several diseases (Rose-John & Heinrich, 1994).

13150 Biochemistry, Vol. 34, No. 40, 1995

Ancian et al.

REFERENCES

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Ids],cpm'x100pl FIGURE4: Effect of varying concentrations of the soluble rabbit M-type sPLA2 receptor on the activity of different sPLA2s. (A) Stoechiometric inhibition of OS2 enzymatic activity by the soluble rabbit M-type sPLA2 receptor. OSz (5 nM) was preincubated during 45 min with increasing concentrations of the soluble rabbit M-type sPLA2 receptor after which [3H]oleicacid labeled E. coli membranes were added in a negligible volume. (B) Inhibition of the enzymatic activities of different sPLA2s. The porcine pancreatic, the bee venom sPLA2s (0.5 nM), and the human group I1 sPLA2 (0.3 nM) were preincubated for 90 min at 25 "C with increasing concentrations of the soluble rabbit M-type sPLA2 receptor after which [3H]oleic acid labeled E. coli membranes were added in a negligible volume. 100% corresponds to the activity of the different sPLA2s measured without inhibitor. 0% represents the nonspecific hydrolysis of labeled phospholipids measured without sPLA2s and represented less than 5% of the total phospholipids. Serum-free medium from mock-transfected cells was without effect on the porcine pancreatic and the human group I1 sPLA2 activities (not shown). (C) Doublereciprocal analysis of the human group I1 sPLA2 activity. Hydrolysis of [3H]oleic acid labeled E. coli phospholipids was measured with the human group I1 sPLA2 at 0.3 nM in the absence ( 0 )or in the presence of 1 (0)or 2 nM (W) soluble M-type sPLAz receptor. The K, value for this inhibition was found to be 1.4 nM. V,,, and K, values without inhibitor for this sPLA2 were 1.14 x lo5 cpm-I min-I (0.1 pmol)-l and 2.7 x lo5 c p d 1 0 0 pL, respectively.

ACKNOWLEDGMENT We thank Dr. Ruth Kramer (Eli Lilly Co.) for the gift of human group I1 sPLA2. We are grateful t o Dr. J.-P. Hugnot for helpful discussions and t o F. Aguila, A . Douy, M.-M. Larroque, and C. Roulinat for their skillful assistance.

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